The U.S. and its allies have made it clear that Syrian strongman Bashar Assad can continue to slaughter his fellow citizens with lead bullets and iron bombs – but that chemical weapons constitute a red line that will trigger a military response from the U.S. and others if he crosses it.

U.S. intelligence officials reported Monday that the separate ingredients to create the nerve agent sarin were being combined as a possible indicator the increasingly-desperate Damascus dictator may be preparing to use them against rebel forces.

“Today, only seven states have not acceded to the CWC [Chemical Weapons Convention]: Angola, Egypt, Israel, Myanmar, North Korea, Somalia, and Syria,” a 2010 Pentagon survey of chemical-weapons states said. “Of those seven, Syria and North Korea most evidently maintain active offensive CW programs.”

Syria has repeatedly said it would not use chemical weapons against its own people, but officials have suggested they might be used to deter outside aggression. The line grows blurry as increasing numbers of rebels battling the Syrian government come from outside the country. There are growing concerns inside the U.S. government that such weapons could fall into the hands – or be given to – terror groups like Hezbollah to use against Israel or U.S. targets in the region.

President Obama warned Syria against using chemical weapons on Monday. “Today I want to make it absolutely clear to Assad and those under his command: The world is watching,” Obama said. “The use of chemical weapons is and would be totally unacceptable. And if you make the tragic mistake of using these weapons, there will be consequences and you will be held accountable.”

Of course, Obama said more than a year ago that it was time for Assad to step down, words that Assad has felt free to ignore. The two-year old rebellion has killed an estimated 40,000 people since March 2011. Why should Assad listen to Obama and others imploring him not to use his Russian-provided chemical weapons?

“There are no down sides to using chemical weapons if the regime is in danger of collapse,” former Defense Intelligence Agency analyst John McCreary says in his NightWatch blog. “Assad is damned if he uses them and might be dead if he does not.”

But these are the calculations Assad, and his shrinking circle of military and political advisers, must now be making:

— First, he has been able to wage war against the rebels – killing many civilians in the process – specifically because he has refrained from using chemical weapons. Using them would force the world community from the sidelines and likely lead in short order to the collapse of what’s left of Assad’s government.

Syria got its first chemical weapons from Egypt in 1972, with Soviet help starting the following year, according to a 1990 U.S. Army assessment. “Syria is believed to have manufactured a number of chemical warheads for its SCUD-B and SS-21 missiles as well as significant stocks of artillery projectiles and aircraft bombs,” it added. “Syria is considered the most advanced Arab country in chemical warfare.”

In 2007, an explosion at a secret military facility in Aleppo reportedly killed 15 people. Syrian officials eventually acknowledged that the explosion occurred while attempting to develop a Scud-C missile with a warhead filled with mustard gas, which is banned under international law. Most of those killed in the blast died as a result of exposure to VX and Sarin nerve agents and mustard blister agents. According to Janes Defence Weekly: “Other Iranian engineers were seriously injured with chemical burns to exposed body parts not protected by safety overalls.”

— Chemical weapons are, fundamentally lousy weapons. Once released into the atmosphere, by spraying, bombing or other means, they float on the winds, which are subject to change. Blowback – poisoning your own troops, friendly forces, or innocent civilians – can be a real problem. But delivering such weapons by missile or air – in other words, from far away – can minimize blowback.

But desperate times call for desperate measures, and it’s fair to suggest that a dictator struggling to hold on to his family dynasty – remember, his father, Hafez, ruled Syria for 29 years before his death in 2000 – may not be thinking clearly.

Senator Richard Lugar noted Monday that a little chemical weapon can go a long way: “The Russians rather confidently told us on one occasion, they took one of the larger shells and they said, `Now, if we set that off in one of your football stadiums under the right circumstances, it would kill all 80,000 people in the stadium.’”

— If it becomes clear that Syria intends to use chemical weapons, there are U.S. special operations forces now in Jordan that would lead an international effort to secure those poisons to prevent their use. The bulk of the weapons are believed to be stored at some 75 sites, concentrated around Al Safira, Damascus, Hamah, Homs and Latakia.

Air strikes against the stockpiles of chemical weapons or their precursors also could be used to try to prevent their use, but that raises concerns that the deadly agents might be dispersed in any such attack.